In automated factories of food or the like, products manufactured in a manufacturing line are transported to a packaging line using a transportation apparatus such as a conveyor, and packaged products are shipped, but quality inspection of the products is carried out between packaging and shipment.
In this quality inspection, as an apparatus that detects foreign metal contained in the products, a metal detection apparatus has been used in the related art. In the metal detection apparatus, basically, a magnetic field for inspection is generated in a region through which an inspection object passes (for example, on the conveyor), and a change in the magnetic field caused by foreign metal contained in a product passing through the region is detected using a magnetic sensor such as a coil or a Hall element.
As described above, strict inspection of foreign substances is carried out on product-manufacturing sites, but supermarkets (hereinafter, abbreviated as SMKT), convenience stores (hereinafter, abbreviated as CVS), and the like employ a system allowing customers to freely touch and select products on display, and thus it is possible to intentionally insert metal such as a needle into commercially available products, and, when the above-described unexpected foreign metal-containing commercially available product is purchased, damage is caused to the customer who purchased this product, and the credit of the store significantly drops.
Therefore, SMKT, CVS, and the like investigate the introduction of an unexpected foreign metal detection apparatus capable of inspecting whether or not a metal foreign substance is contained in a commercially available product when the customer makes a payment for the commercially available product brought to a counter with an intention of purchasing the product.
As a metal detection apparatus that can be employed in SMKT, CVS, and the like, Patent Document 1 describes an example in which a flat metal detection apparatus is used on a counter table in a counter.
In the metal detection apparatus disclosed by Patent Document 1, an upper plate 121 on which an article is place is supported by an upper surface of a counter table in a counter, a transmission coil 123 is wound around the outermost circumference of a rectangular head frame 122 fixed to a lower surface side of the upper plate 121, two receiving coils 124 and 125 having the same bore are disposed side by side in the transmission coil 123, and magnetic fluxes generated by the transmission coil 123 are almost equally interlinked with the receiving coils 124 and 125.
Therefore, when an article placed on the upper plate 121 has metal contained therein, a difference is generated between the interlinked magnetic fluxes of the two receiving coils 124 and 125 due to the influence of the metal on the magnetic field, and whether or not metal is contained in a commercially available product can be determined by detecting the difference in the magnetic fluxes.